Fluidized bed heat exchanger

ABSTRACT

A vapor generator in which a plurality of vertically stacked beds of particulate material containing a solid fuel are disposed in a housing. Air is passed through each of the fuel beds to promote the combustion of the fuel and maintain the beds at predetermined temperatures while a heat exchange medium is circulated in a heat exchange relation to the beds.

United States Patent 1191 Bryers et al. July 16, 1974 FLUIDIZED BED HEATEXCHANGER 2,983,259 5/1961 Wittke 122/4 1 1 Richard William Briers,North 32131333 51323 2Z5i1i.'.:::.... 31331313331133: iii/Z Cranford;Jack David Sheflker, 3,648,666 3/1972 Foldes etal. 122/4 Dover, both ofNJ. 3,659,559 5/1972 Foldes et a]. 122/4 Assigneez The United States oAmerica as 3,736,908 6/1973 Ehrlich et al. l22/4 represented by theUnited States Environmental Protection Agency, ExammeF-Kenneth SpragueWashington, DC, Attorney, Agent, or Firm-Marvin A. Naigur; John E.Wilson [22] F1led: Jan. 16, 1973 [21] Appl. No.: 324,04l [57] ABSTRACT52 us. (:1. 122/4 11), 110/28 J A generator i which a plurality 9Yeftically 51 Int. Cl F22b 1/02 g d ff g. P xf" f f g 58 Id u ue areispose in a ousmg. 1r 1s passe t roug 1 m of Search 122/4' 110/28 J eachof the fuel beds to promote the combustion of 15613251221112111222;321:2231?1221121221 2 818 049 gl t k PATENTS in a heatexchange relation to the beds.

as ows i l224X 1 2,842,102 7/1958 Blaskowski 122/4 9 Claims, 2 DrawingFigures PATENTEU JUL 1 s 1974 saw 1 a? 2 3: Iii

PATENTEDJUU 61m FLUIDIZED BED HEAT EXCHANGER BACKGROUND OF THE INVENTIONThis invention relates to a fluidized bed heat exchanger, and moreparticularly, to a steam generator which consists of a plurality ofstacked fluidized beds for generating heat.

The use of a low-grade solid fuel, such as coal, is a well-known sourceof heat in the use of generation of steam. In some of these arrangementsthe fuel is disposed in a fixed bed with a chain grate stoker or thelike utilized to promote its combustion, and'water is passed in a heatexchange relation thereto to produce the steam. However, thesearrangements suffer from several disadvantages including problems inhandling the solid fuel while adding it to or removing it from the bedsduring operation. Also, a relatively low heat transfer is achieved andthe bed temperatures are often nonuniform and hard to control.

Attempts have been made to utilize a fluidized bed to produce heat forgenerating steam due to the fact that a fluidized bed enjoys theadvantages of an improved heat transfer rate, a reduction in corrosion,a reduction in boiler fouling, an increase in combustion efficiency,combustion at lower temperatures and a reduction in boiler size. Inthese arrangements, air is passed upwardly through a mass of particulatefuel material causing the material to expand and take on a suspended orfluidized state. However, there is a inherent limitation on the range ofheat input to the water passing in a heat exchange relation to thefluidized bed, largely due to the fact that the quantity of air suppliedto the bed must be sufficient to maintain same in a fluidized conditionyet must not cause excessive quantities of the fuel material to be blownaway.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide a heat exchanger which enjoys the advantages of thefluidized bed yet enables a relatively large range of heat transfer tobe obtained.

It is a further object of the present invention to provide a heatexchanger of modular construction which includes a plurality of stackedfluidized beds yet which can be manufactured with a minimum ofcomponents in a relatively simple manner.

Toward the fulfillment of these and other objectives,

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevational viewof the steam generator of the present invention; and

FIG. 2 is a top plan view showing a portion of the steam generator ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring specifically to FIG.1 of the drawings, the

reference numeral 10 refers in general to a housing having severalopenings for the passage of air therethrough and for receiving tubes aswill be described in detail later. An enclosure 12 is defined within thehousing and comprises a front wall 14 and a rear wall 16 shown in crosssection, with each wall being formed by a plurality of finned tubeswelded together in a conventional manner and extending for the entirelength of the wall. A pair of side walls, identical to the front wall 14and rear wall 16, are also provided but are not shown in FIG. 1 for theconvenience of presentation.

A pair of headers, shown in end view by the reference numerals 18 and20, are provided at the top of the enclosure 12, along with a header 22,it being understood that another header is disposed behind the header 22as viewed in FIG. 1. In a similar manner, headers 24, 26, and 28 aredisposed at the bottom of the enclosure 12 with it being understood thatan additional header is disposed behind the header 28.

A plurality of horizontal, perforated air distribution plates 30 aredisposed in a spaced relation in the enclosure 12 to divide theenclosure into a plurality of vertically stacked compartments, whichdefine beds, designated by five reference numerals 31.

An air plenum chamber 32 extends below each of the plates 30 fordistribution of air to the beds 31. Particulate fuel is fed as a densephase mixture of air and fuel by means of a multiplicity of feed lines34 which are associated with each of the beds 31. The feed lines 34extend through a suitable opening provided in the rear wall 16, and passthrough the plenum chamber 32 and air distributor plate 30 into bed 31where the coal is discharged. The inlets 34 are adapted to receive thefuel in a conventional manner from a source such as a pneumatic feeder,which has not been shown in the drawings for the sake of simplicity.

A series of tubes, shown in general by the reference numeral 36, aredisposed in the enclosure 12 and extend from an inlet 38 upwardly forthe entire length of .the enclosure 12 in a serpentine relationship toform a plurality of banks respectively disposed in the zone above thefluid bed 31 in an area where heat is primarily transferred byconvection. Although only a single tube 36 is shown diagrammatically inFIG. 1, it is understood that a plurality of juxtapositioned tubes areprovided, forming a tube bundle, that extends across the entire width ofthe enclosure 12.

A header 40 is disposed at the top of the enclosure 12 and registerswith the tube bundle 36. As a result, a heat exchanger medium, such aswater, passing in through the inlet 38 from a boiler feed pump, or thelike, passes through the various banks of the tube bundle 36 whereby itis' gradually heated before entering the header 40 for furtherdistribution, as will be described in detail later.

A plurality of feeder tubes 42, 44, and 46 are connected'to the headers18, 40, and 20, respectively, at

the upper-portion of the enclosure 12 while a plurality of feeder tubes48-and 49 are connected tothe headers 24 and 26, respectively at thelower portion of the enclosure. Although not shown in the drawings, itis understood that additional feeder tubes are provided which areconnected to the headers 22 and 28 and the other two headers not shownin the drawings, as discussed above. v

Each feeder tube discussed above is connected to a respective downcomerconduit, one of which is shown by the reference numeral '50, it beingunderstood that several additional downcomer conduits extendimmediately-behind the downcomer conduit 50 as viewed in FIG. '1 and aresimilar thereto.

A pair of tube bundles 52 and 54 are disposed in adjacent compartmentswithin the enclosure 12 and are connected in series between a pair ofheaders 56 and 58, respectively, which, in turn, are connected by themeans of feeder tubes to separate downcomer conduits, similar to andextending behind the downcomer conduit 50.

a In a similar manner, an additional pair of tube bundles 60 and 62 aredisposed in adjacent compartments above the tube bundles 52 and 54 andare connected in series via a' header 63. The tube bundle 60 isconnected to a downcomer conduit extending to the rear of the downcomerconduit 50 via a header 64 and the tube bundle 62 is connected via aheader 66 to an outlet conduit 68.

A tube bundle 70 is provided in the uppermost compartmentin theenclosure 12 and is connected to an inlet conduit 72 via a header 73.and to an outlet conduit 74 via a header 75. It is noted that each tubebundle 52, 54, 60, 62, and 70 is submerged in its respective fluidizedbed to effect a heat transfer of liquid passing therethrough as will bedescribed in detail later.

A damper controlled air inlet 80 is provided adjacent each plenumchamber 32 for passing air in the directions indicated bythe solidarrows in FIG. 1 through beds 31 of particulate material and fuel tofluidize the beds 31 in a conventional manner, it being understood thatthe velocity and rate of flow of the air passing through the bedsisregulated so that it is high enough to fluidize the particulate fueland to obtain economical burning or heat release rates per unit area ofbed, yet is low enough to avoid the loss of too many fine fuel particlesfrom the bed and to allow sufficie'nt residence time of gases for goodsulphur removal by a sorbent added to the fuel as also will bedescribedin detail later.

The heated air, after passing through the fluidized beds, combines withthe combustion products from the beds and the resulting mixture or, gas,exits through outlets 82 provided in the-rear wall 16 as shown by thedashed arrows, where it flows into a chamber 84 disposed to the rear ofthe wall 16. The gas is directed from the chamber 84, through a duct 86and to a cyclone type dust collector 90 which removes the fine coalparticles entrained in the gas. Referring to FIGS. 1 and 2, the cleangas with the fines removed is then passed via a duct 92 to a tubular airheater shown in general by the reference numeral 94. This air heatercomprises a series of tubes 96 for receiving the clean gas and directingsame downwardly as shown by the dotted arrows in FIGJI where it exitsthrough an outlet Air from an external source enters the system throughan inlet 100 where it passes through a duct 102 adjacent the tubular airheater 94 and in a vertical direction as shown by'the solid arrowswhereby it is preheated. From the top of the duct 102 the preheated airis directed via ducts 104, 106, and 108 to the housing whereby it isseparated into the five separate streams of air entering the air inlets80.

Referring again to FIG. 1, after being separated out of the gas streamby the dust collector 90, the fine parplenums 32 through the' ticulatefuel material is directed to a dust hopper 110 and then into an injector112 which injects the fines back into the lowest compartment in theenclosure 12 whereby it is fluidized and burned in a similar manner tothe remaining fluidized beds. Air passing through this latter fluidizedbed exits into an air chamber 120 adjacent the chamber 84 and isdirected via a separate duct 122 (FlG. 2) to the tubular air heater 94.

In operation, each bed is started up by firing an auxiliary gas burneror the like (not shown) to the minimum fuel ignition temperature wherebyfuel will be injected and combusted and each bed-will continue burningafter startup. The heat exchange medium, such as water, is introducedinto the inlet 38 whereby it passes in series through each of the tubebundles 36 to raise its temperature to a predetermined level. It thenpasses from the uppermost tube bundle 36 to the header 40 and then fromthe feeder tubes 44 to the downcomer conduit '50 where it is directedinto the header 56. From the header 56the'water passes in series throughthe tube bundles 52 and 54 whereby it is partially'evaporated beforeexiting via'a header 58 to a downcomer conduit located immediately tothe rear-of the downcomer conduit 50. The latter downcomer conduitdifeeder tubes 48 and to'the header 24 whereby the mixtu'r'e is passedupwardly through the finned tube wall 14 for the entirelength thereof tofurther raise the temperature of the mixture. The mixture is thencollected in the header 18, and fed, by means of the feeder tubes 42, toanother additional downcomer conduit similar to downcomer conduit 50 andlocated therebehind, whereby it is directed to the sidewall header 28via the feeder tubes associated therewith.

The water-steam mixture then passes completely up thelatter sidewall tothe header. 22 where it is fed, .via the associated feeder tubes toanother downcomer conduit similar to the downcomer conduit 50 andextending therebehind, whereby it is directed to and through 1 the othersidewall and the rear wall 16 in an identical manner. During the passagethrough the four walls of the enclosure 12, complete evaporation of thewater into steam takes place. i

After passing through the final wall 16, the steam is collected intheheader. 20 and passed, via the feeder tubes 46 to still anotherdowncomer conduit where it is then passed, via a header 64, to andthrough the tube bundle 60, the header '63, and the tube bundle 62thereby raising the temperature of the steam to superheat. Thesuperheated steam is then collected in a header 66 and passed outthrough the outlet 68 where low temperature steam which has previouslybeen used in another stage of the plant such as a steam turbine, toraise its temperature for further use. 'In particular, thelatter steamis received by an inlet 72 and passed, via a header 73, through thesteam bundle to raise the temperature of the steam before it exits via aheader 73 and an outlet 74.

According to a preferred embodiment the particulate fuel is in the formof a mixture ofcrushed bituminous coal and limestone, with. the latterfunctioning as a sorbent for the, sulphur dioxide in combustion gasesfrom the coal in accordance with conventional chemical theory. Since thelow combustion temperatures and the low excess air requirements alsoreduce the nitrogen oxide from the combustion gas, the latter contains aminimum of pollutants.

There are many other advantages of the arrangement of the presentinvention. For example, the use of the vertical stacked compartmentsdefined by continuous walls considerably reduces the manufacturing costsand time, since it minimizes headers, interconnecting piping, anddowncomers yet permits a maximum use of the heat transfer surfacesinvolved. Of course, the free movement of the particulate fuel in thefluidized bed promotes rapid heat transfer both within the bed andbetween the bed and the submerged tube banks. As a result, bedtemperatures are uniform and easy to control.

The cost of construction is reduced by minimizing boiler cross sectionalarea and maximizing the number of components that are shop fabricated,such that the boiler dimensions can meet shipping, dimensional andweight limitations. Also, the start-up procedures are greatly simplifiedby assigning only one heating function to each bed, such that no bedmust be started with uncooled tubes. Thus, the evaporating beds arestarted first with circulating water, and superheating beds are startedlast after steam has been generating. The startup time is also reduced,and the heat required in the form of preheating ignitors is reduced asthe flue gas from the evaporating beds preheats the air to thesuperheater. The assigning of separate heating functions to theindividual beds also simplifies and improves steam temperature controlby differential firing of coal to each bed. The modular constructionsimplifies load control, and a four to one turn down can be achieved bysimply shutting downthree modules. This also improves on load time asindividual modules can be serviced without loosing the the entire boilersystem. Also, the economizer in the zone above the bed reduces gastemperature to nearly the same level as the air inlet temperature andwater wall enclosure temperature, thereby minimizing differences inexpansion of pressure parts or heat exchange components.

It should also be noted that a vertically stacked bed with a fin-tubewater wall construction simplifies the circuitry and minimizes thenumber. of headers, downcomers and feeder pipes. Thus, the fin-tubewater wall construction has the following attendant advantages:

a. Provides a support for heat transfer surface, pressure parts andfluid beds.

b. Protects enclosure wall or shell from high temperature gases.

0. Provides heat transfer surface thereby maximizing the utility of allcomponents to serve their primary function of heat exchange.

d. Provides a partition between the flue gas and inlet air.

e. Reduces the surface requirements in the bed.

f. Reduces cost by employing shop fabrication techniques.

g. Reduces maintenance cost.

Still other advantagesof the heat exchanger of the present inventioninclude reduction in the corrosion of the tubes, etc., due to therelatively low combustion temperatures available and a reduction incosts since cheaper construction materials can be used by virtue of thehigh heat transfer rates at the relatively low temperatures.

It is understood that the compactness and operation of the heatexchanger lends itself to incorporation in a modular system, since fouror five units described above can be utilized in a side by siderelation.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed with a corresponding use of other features.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the spirit and scope of theinvention herein.

What is claimed is:

1. A heat exchanger comprising a housing, a plurality of verticallyspaced beds of particulate fuel material disposed in said housing anddefining a plurality of heat zones, means for passing air through eachof said fuel beds to promote the combustion of said fuel and maintainsaid heat zones at predetermined temperatures, and means forsuccessively passing a heat exchange medium upwardly through said heatzones in a heat exchange relation to said fuel beds to gradually raisethe temperature of said medium.

2. The heat exchanger of claim 1 wherein each wall of said housing isformed by. a plurality of continuous finned tubes for circulating saidheat exchange medium.

3. The heat exchanger of claim 1 further comprising means to selectivelydirect said medium through at least one of said fuel beds after it haspassed upwardly through all of said heat zones.

4. The heat exchanger of claim 3 wherein said medium is water, saidwater being partially evaporated during passage through said heat zonesand being completely evaporated into steam during passage through saidfuel bed.

5. The heat exchanger of claim 4-further comprising means to selectivelydirect said steam through at least one of said fuel beds to superheatthe steam.

6. The heat exchanger of claim 1 further comprising means to direct saidmedium outwardly from and back into said housing after it has passedthrough said heat zones.

7. The heat exchanger of claim 1 wherein said air combines with thecombustion products from said fuel .beds as it passes through said fuelbeds to form a heated gas, and further comprising means to direct saidheated gas in a heat exchange relation to said air before it passesthrough said fuel beds.

8. The heat exchanger of claim 7 wherein a portion of said fuel materialis entrained in said gas, and further comprising means to separate saidportion of fuel material from said gas and direct said portion of fuelmaterial to one of said fuel beds.

9. The heat exchanger of claim 1 wherein said heat zones are locatedimmediately above their respective beds.

1. A heat exchanger comprising a housing, a plurality of verticallyspaced beds of particulate fuel material disposed in said housing anddefining a plurality of heat zones, means for passing air through eachof said fuel beds to promote the combustion of said fuel and maintainsaid heat zones at predetermined temperatures, and means forsuccessively passing a heat exchange medium upwardly through said heatzones in a heat exchange relation to said fuel beds to gradually raisethe temperature of said medium.
 2. The heat exchanger of claim 1 whereineach wall of said housing is formed by a plurality of continuous finnedtubes for circulating said heat exchange medium.
 3. The heat exchangerof claim 1 further comprising means to selectively direct said mediumthrough at least one of said fuel beds after it has passed upwardlythrough all of said heat zones.
 4. The heat exchanger of claim 3 whereinsaid medium is water, said water being partially evaporated duringpassage through said heat zones and being completely evaporated intosteam during passage through said fuel bed.
 5. The heat exchanger ofclaim 4 further comprising means to selectively direct said steamthrough at least one of said fuel beds to superheat the steam.
 6. Theheat exchanger of claim 1 further comprising means to direct said mediumoutwardly from and back into said housing after it has passed throughsaid heat zones.
 7. The heat exchanger of claim 1 wherein said aircombines with the combustion products from said fuel beds as it passesthrough said fuel beds to form a heated gas, and further comprisingmeans to direct said heated gas in a heat exchange relation to said airbefore it passes through said fuel beds.
 8. The heat exchanger of claim7 wherein a portion of said fuel material is entrained in said gas, andfurther comprising means to separate said portion of fuel material fromsaid gas and direct said portion of fuel material to one of said fuelbeds.
 9. The heat exchanger of claim 1 wherein said heat zones arelocated immediately above their respective beds.